{"id":2747,"date":"2018-06-21T13:25:00","date_gmt":"2018-06-21T13:25:00","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/chapter\/8-5-leaving-groups-2\/"},"modified":"2021-12-08T15:13:06","modified_gmt":"2021-12-08T15:13:06","slug":"8-6-assessing-sn1-sn2-e1-e2-which-will-happen","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/chapter\/8-6-assessing-sn1-sn2-e1-e2-which-will-happen\/","title":{"raw":"8.6. Assessing SN1, SN2, E1, E2: Which will happen?","rendered":"8.6. Assessing SN1, SN2, E1, E2: Which will happen?"},"content":{"raw":"<section class=\"mt-content-container\">\r\n<div id=\"section_4\" class=\"mt-section\">\r\n<h1>More detailed analysis<\/h1>\r\n<div id=\"s14800\" class=\"mt-include\">\r\n\r\n<strong>The most important factors to consider are the structure of the electrophilic alkyl group and the nature of the nucleophilic reactant<\/strong>.\u00a0 In general, in order for an SN1 or E1 reaction to occur, the relevant carbocation intermediate must be relatively stable.\u00a0 Strong nucleophiles favor substitution, and strong bases, especially strong hindered bases (such as tert-butoxide) favor elimination.\r\n\r\nThe nature of the halogen substituent on the alkyl halide is usually not very significant if it is Cl, Br or I. In cases where both S<sub>N<\/sub>2 and E2 reactions compete, chlorides generally give more elimination than do iodides, since the greater electronegativity of chlorine increases the acidity of beta-hydrogens. Indeed, although alkyl fluorides are relatively unreactive, when reactions with basic nucleophiles are forced, elimination occurs (note the high electronegativity of fluorine).\r\n<div class=\"textbox shaded\">\r\n\r\nGeneral guidelines\r\n<ul>\r\n \t<li>Leaving group (usually a halogen) is required \u2013 if none is present, none of these reactions occurs!<\/li>\r\n \t<li>1<sup>o<\/sup> favors S<sub>N<\/sub>2 or E2, never S<sub>N<\/sub>1 or E1 (unless resonance can happen). 3<sup>o<\/sup> easily does E2, never S<sub>N<\/sub>2.<\/li>\r\n \t<li>sp<sup>2<\/sup> C-X compounds (e.g., aryl halides) do not undergo nucleophilic substitution, and eliminations are difficult.<\/li>\r\n \t<li>Strong base\/nucleophile =&gt; S<sub>N<\/sub>2 or E2<\/li>\r\n \t<li>Weak base\/nucleophile =&gt; S<sub>N<\/sub>1 or E1<\/li>\r\n \t<li>Polar aprotic solvent helps S<sub>N<\/sub>2; polar protic helps S<sub>N<\/sub>1\/E1.<\/li>\r\n \t<li>Heat favors elimination (E1\/E2), cold favors substitution (S<sub>N<\/sub>1\/S<sub>N<\/sub>2)<\/li>\r\n<\/ul>\r\n<\/div>\r\nThe following tables summarize the expected outcomes of alkyl halide reactions with two common classes of oxygen nucleophiles: <strong>Strong<\/strong> (hydroxide, \u00afOH or alkoxides, \u00afOR') and <strong>weak<\/strong> (water, H<sub>2<\/sub>O or alcohols, R'OH). It is assumed that the alkyl halides have one or more beta-hydrogens, making elimination possible; and that polar solvents are used.\r\n<table style=\"width: 524px;\" border=\"1\">\r\n<tbody>\r\n<tr>\r\n<td style=\"width: 249.567px;\"><span style=\"background-color: #ffff00;\"><strong>Primary alkyl halide R-X<\/strong><\/span><\/td>\r\n<td style=\"width: 164.383px;\"><strong>Cold<\/strong><\/td>\r\n<td style=\"width: 108.25px;\"><strong>Hot<\/strong><\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 249.567px;\"><strong>Strong (reaction with \u00afOH or \u00afOR')<\/strong><\/td>\r\n<td style=\"width: 164.383px;\"><strong>S<sub>N<\/sub>2\r\n<\/strong><\/td>\r\n<td style=\"width: 108.25px;\"><strong>E2, some S<sub>N<\/sub>2\r\n<\/strong><\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 249.567px;\"><strong>Weak (reaction with H<sub>2<\/sub>O or R'OH)<\/strong><\/td>\r\n<td style=\"width: 164.383px;\"><strong>V. slow S<sub>N<\/sub>2 or no reaction\r\n<\/strong><\/td>\r\n<td style=\"width: 108.25px;\"><strong>Slow E2 or S<sub>N<\/sub>2\r\n<\/strong><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<table style=\"width: 524px;\" border=\"1\">\r\n<tbody>\r\n<tr>\r\n<td><span style=\"background-color: #ffff00;\"><strong>Secondary alkyl halide R-X<\/strong><\/span><\/td>\r\n<td><strong>Cold<\/strong><\/td>\r\n<td><strong>Hot<\/strong><\/td>\r\n<\/tr>\r\n<tr>\r\n<td><strong>Strong (reaction with \u00afOH or \u00afOR')<\/strong><\/td>\r\n<td><strong>S<sub>N<\/sub>2, possibly some E2\r\n<\/strong><\/td>\r\n<td><strong>E2\r\n<\/strong><\/td>\r\n<\/tr>\r\n<tr>\r\n<td><strong>Weak (reaction with H<sub>2<\/sub>O or R'OH)<\/strong><\/td>\r\n<td><strong>Slow S<sub>N<\/sub>1<\/strong><\/td>\r\n<td><strong>Slow S<sub>N<\/sub>1 or E1<\/strong><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<table style=\"border-collapse: collapse; width: 524px; height: 42px;\" border=\"1\">\r\n<tbody>\r\n<tr style=\"height: 14px;\">\r\n<td style=\"width: 20.7247%; height: 14px;\"><span style=\"background-color: #ffff00;\"><strong>Tertiary alkyl halide R-X<\/strong><\/span><\/td>\r\n<td style=\"width: 13.9892%; height: 14px;\"><strong>Cold<\/strong><\/td>\r\n<td style=\"width: 9.79141%; height: 14px;\"><strong>Hot<\/strong><\/td>\r\n<\/tr>\r\n<tr style=\"height: 14px;\">\r\n<td style=\"width: 20.7247%; height: 14px;\"><strong>Strong (reaction with \u00afOH or \u00afOR')<\/strong><\/td>\r\n<td style=\"width: 13.9892%; height: 14px;\"><strong>E2, possibly some S<sub>N<\/sub>1\r\n<\/strong><\/td>\r\n<td style=\"width: 9.79141%; height: 14px;\"><strong>E2<\/strong><\/td>\r\n<\/tr>\r\n<tr style=\"height: 14px;\">\r\n<td style=\"width: 20.7247%; height: 14px;\"><strong>Weak (reaction with H<sub>2<\/sub>O or R'OH)<\/strong><\/td>\r\n<td style=\"width: 13.9892%; height: 14px;\"><strong>S<sub>N<\/sub>1, possibly E1<\/strong><\/td>\r\n<td style=\"width: 9.79141%; height: 14px;\"><strong>E1<\/strong><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<div class=\"mt-section\">\r\n<p class=\"editable\">Note that some anionic nucleophiles are less basic than \u00afOH\/OR', such as acetate CH<sub>3<\/sub>COO\u00af (weakly basic) or iodide (non-basic).\u00a0 These will tend to give more substitution and much less elimination.<\/p>\r\nAmmonia (NH<sub>3<\/sub>) and amines (usually RNH<sub>2<\/sub> or R<sub>2<\/sub>NH), are common uncharged nitrogen nucleophiles.\u00a0 These are more powerful nucleophiles than water or alcohols, but they are also basic, leading to more elimination in cases where this is possible.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/section>","rendered":"<section class=\"mt-content-container\">\n<div id=\"section_4\" class=\"mt-section\">\n<h1>More detailed analysis<\/h1>\n<div id=\"s14800\" class=\"mt-include\">\n<p><strong>The most important factors to consider are the structure of the electrophilic alkyl group and the nature of the nucleophilic reactant<\/strong>.\u00a0 In general, in order for an SN1 or E1 reaction to occur, the relevant carbocation intermediate must be relatively stable.\u00a0 Strong nucleophiles favor substitution, and strong bases, especially strong hindered bases (such as tert-butoxide) favor elimination.<\/p>\n<p>The nature of the halogen substituent on the alkyl halide is usually not very significant if it is Cl, Br or I. In cases where both S<sub>N<\/sub>2 and E2 reactions compete, chlorides generally give more elimination than do iodides, since the greater electronegativity of chlorine increases the acidity of beta-hydrogens. Indeed, although alkyl fluorides are relatively unreactive, when reactions with basic nucleophiles are forced, elimination occurs (note the high electronegativity of fluorine).<\/p>\n<div class=\"textbox shaded\">\n<p>General guidelines<\/p>\n<ul>\n<li>Leaving group (usually a halogen) is required \u2013 if none is present, none of these reactions occurs!<\/li>\n<li>1<sup>o<\/sup> favors S<sub>N<\/sub>2 or E2, never S<sub>N<\/sub>1 or E1 (unless resonance can happen). 3<sup>o<\/sup> easily does E2, never S<sub>N<\/sub>2.<\/li>\n<li>sp<sup>2<\/sup> C-X compounds (e.g., aryl halides) do not undergo nucleophilic substitution, and eliminations are difficult.<\/li>\n<li>Strong base\/nucleophile =&gt; S<sub>N<\/sub>2 or E2<\/li>\n<li>Weak base\/nucleophile =&gt; S<sub>N<\/sub>1 or E1<\/li>\n<li>Polar aprotic solvent helps S<sub>N<\/sub>2; polar protic helps S<sub>N<\/sub>1\/E1.<\/li>\n<li>Heat favors elimination (E1\/E2), cold favors substitution (S<sub>N<\/sub>1\/S<sub>N<\/sub>2)<\/li>\n<\/ul>\n<\/div>\n<p>The following tables summarize the expected outcomes of alkyl halide reactions with two common classes of oxygen nucleophiles: <strong>Strong<\/strong> (hydroxide, \u00afOH or alkoxides, \u00afOR&#8217;) and <strong>weak<\/strong> (water, H<sub>2<\/sub>O or alcohols, R&#8217;OH). It is assumed that the alkyl halides have one or more beta-hydrogens, making elimination possible; and that polar solvents are used.<\/p>\n<table style=\"width: 524px;\">\n<tbody>\n<tr>\n<td style=\"width: 249.567px;\"><span style=\"background-color: #ffff00;\"><strong>Primary alkyl halide R-X<\/strong><\/span><\/td>\n<td style=\"width: 164.383px;\"><strong>Cold<\/strong><\/td>\n<td style=\"width: 108.25px;\"><strong>Hot<\/strong><\/td>\n<\/tr>\n<tr>\n<td style=\"width: 249.567px;\"><strong>Strong (reaction with \u00afOH or \u00afOR&#8217;)<\/strong><\/td>\n<td style=\"width: 164.383px;\"><strong>S<sub>N<\/sub>2<br \/>\n<\/strong><\/td>\n<td style=\"width: 108.25px;\"><strong>E2, some S<sub>N<\/sub>2<br \/>\n<\/strong><\/td>\n<\/tr>\n<tr>\n<td style=\"width: 249.567px;\"><strong>Weak (reaction with H<sub>2<\/sub>O or R&#8217;OH)<\/strong><\/td>\n<td style=\"width: 164.383px;\"><strong>V. slow S<sub>N<\/sub>2 or no reaction<br \/>\n<\/strong><\/td>\n<td style=\"width: 108.25px;\"><strong>Slow E2 or S<sub>N<\/sub>2<br \/>\n<\/strong><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<table style=\"width: 524px;\">\n<tbody>\n<tr>\n<td><span style=\"background-color: #ffff00;\"><strong>Secondary alkyl halide R-X<\/strong><\/span><\/td>\n<td><strong>Cold<\/strong><\/td>\n<td><strong>Hot<\/strong><\/td>\n<\/tr>\n<tr>\n<td><strong>Strong (reaction with \u00afOH or \u00afOR&#8217;)<\/strong><\/td>\n<td><strong>S<sub>N<\/sub>2, possibly some E2<br \/>\n<\/strong><\/td>\n<td><strong>E2<br \/>\n<\/strong><\/td>\n<\/tr>\n<tr>\n<td><strong>Weak (reaction with H<sub>2<\/sub>O or R&#8217;OH)<\/strong><\/td>\n<td><strong>Slow S<sub>N<\/sub>1<\/strong><\/td>\n<td><strong>Slow S<sub>N<\/sub>1 or E1<\/strong><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<table style=\"border-collapse: collapse; width: 524px; height: 42px;\">\n<tbody>\n<tr style=\"height: 14px;\">\n<td style=\"width: 20.7247%; height: 14px;\"><span style=\"background-color: #ffff00;\"><strong>Tertiary alkyl halide R-X<\/strong><\/span><\/td>\n<td style=\"width: 13.9892%; height: 14px;\"><strong>Cold<\/strong><\/td>\n<td style=\"width: 9.79141%; height: 14px;\"><strong>Hot<\/strong><\/td>\n<\/tr>\n<tr style=\"height: 14px;\">\n<td style=\"width: 20.7247%; height: 14px;\"><strong>Strong (reaction with \u00afOH or \u00afOR&#8217;)<\/strong><\/td>\n<td style=\"width: 13.9892%; height: 14px;\"><strong>E2, possibly some S<sub>N<\/sub>1<br \/>\n<\/strong><\/td>\n<td style=\"width: 9.79141%; height: 14px;\"><strong>E2<\/strong><\/td>\n<\/tr>\n<tr style=\"height: 14px;\">\n<td style=\"width: 20.7247%; height: 14px;\"><strong>Weak (reaction with H<sub>2<\/sub>O or R&#8217;OH)<\/strong><\/td>\n<td style=\"width: 13.9892%; height: 14px;\"><strong>S<sub>N<\/sub>1, possibly E1<\/strong><\/td>\n<td style=\"width: 9.79141%; height: 14px;\"><strong>E1<\/strong><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"mt-section\">\n<p class=\"editable\">Note that some anionic nucleophiles are less basic than \u00afOH\/OR&#8217;, such as acetate CH<sub>3<\/sub>COO\u00af (weakly basic) or iodide (non-basic).\u00a0 These will tend to give more substitution and much less elimination.<\/p>\n<p>Ammonia (NH<sub>3<\/sub>) and amines (usually RNH<sub>2<\/sub> or R<sub>2<\/sub>NH), are common uncharged nitrogen nucleophiles.\u00a0 These are more powerful nucleophiles than water or alcohols, but they are also basic, leading to more elimination in cases where this is possible.<\/p>\n<\/div>\n<\/div>\n<\/section>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-2747\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>8.5: Leaving groups. <strong>Authored by<\/strong>: Tim Soderbergu00a0(University of Minnesota, Morris). <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry\/Book%3A_Organic_Chemistry_with_a_Biological_Emphasis_(Soderberg)\/Chapter_08%3A_Nucleophilic_substitution_reactions_I\/8.5%3A_Leaving_groups#8.5D:_Predicting_SN1_vs._SN2_mechanisms.3B_competition_between_nucleophilic_substitution_and_elimination_reactions\">https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry\/Book%3A_Organic_Chemistry_with_a_Biological_Emphasis_(Soderberg)\/Chapter_08%3A_Nucleophilic_substitution_reactions_I\/8.5%3A_Leaving_groups#8.5D:_Predicting_SN1_vs._SN2_mechanisms.3B_competition_between_nucleophilic_substitution_and_elimination_reactions<\/a>. <strong>Project<\/strong>: Chemistry LibreTexts. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/4.0\/\">CC BY-NC-SA: Attribution-NonCommercial-ShareAlike<\/a><\/em><\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":311,"menu_order":6,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"8.5: Leaving groups\",\"author\":\"Tim Soderbergu00a0(University of Minnesota, Morris)\",\"organization\":\"\",\"url\":\"https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry\/Book%3A_Organic_Chemistry_with_a_Biological_Emphasis_(Soderberg)\/Chapter_08%3A_Nucleophilic_substitution_reactions_I\/8.5%3A_Leaving_groups#8.5D:_Predicting_SN1_vs._SN2_mechanisms.3B_competition_between_nucleophilic_substitution_and_elimination_reactions\",\"project\":\"Chemistry LibreTexts\",\"license\":\"cc-by-nc-sa\",\"license_terms\":\"\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-2747","chapter","type-chapter","status-publish","hentry"],"part":25,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/2747","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/users\/311"}],"version-history":[{"count":18,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/2747\/revisions"}],"predecessor-version":[{"id":5169,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/2747\/revisions\/5169"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/parts\/25"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/2747\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/media?parent=2747"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapter-type?post=2747"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/contributor?post=2747"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/license?post=2747"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}